Supplementary Information for A 61-Million-Person ... - James Fowler

3 Data Science, Facebook, Inc., Menlo Park, CA 94025, USA ..... Gordon super computer19 at the San Diego Super Computer Center because we simulate a.
6MB Sizes 2 Downloads 80 Views
Supplementary Information for A 61-Million-Person Experiment in Social Influence and Political Mobilization Robert M. Bond1, Christopher J. Fariss1, Jason J. Jones2, Adam D. I. Kramer3, Cameron Marlow3, Jaime E. Settle1, and James H. Fowler1,4* 1

Political Science Department, University of California, San Diego, La Jolla, CA 92093, USA Psychology Department, University of California, San Diego, La Jolla, CA 92093, USA 3 Data Science, Facebook, Inc., Menlo Park, CA 94025, USA 4 Medical Genetics Division, University of California, San Diego, La Jolla, CA 92093, USA 2

∗ To whom correspondence should be addressed, email: [email protected]

TABLE OF CONTENTS Research Design.............................................................................................................................. 2! Distribution of Key Variables and Balance Testing ....................................................................... 2! Matching to Voting Records ........................................................................................................... 3! Overreporting and Underreporting of Voting Behaviour ............................................................... 4! Determination of “Close” Friends .................................................................................................. 5! Correlation in Behaviour Between Friends..................................................................................... 6! Analysis of Direct and Indirect Effects ........................................................................................... 7! Variation in the Treatment Effect by Number of Friends Shown ................................................... 9! Recency of Contact ......................................................................................................................... 9! Average Per-Friend Treatment Effect vs. Number of Friends ........................................................ 9! Monte Carlo Tests of the Network Permutation Method.............................................................. 10! Monte Carlo Code in R ................................................................................................................. 13! Tables ............................................................................................................................................ 15! Figures........................................................................................................................................... 34! SI References ................................................................................................................................ 40!

1

Research Design The research design for this study was reviewed and approved by the University of California, San Diego Human Research Protections Program (protocol #101273). All registered Facebook users over the age of 18 who logged in to their Facebook account on November 2, 2010, were automatically included in the experiment. Random assignment to one of the three treatment groups was done using a random number generator. In total, 61,279,316 Facebook users participated in the study. Most participants (98%) were exposed to the “social message” condition (60,055,176). Half of the remaining participants were exposed to the “informational message” condition (613,096) and the rest were in the control (“no message”) group (611,044). Ideally, we would have designed the experiment with equal sized treatment and control groups to maximize power. However, Facebook wanted to encourage all users to participate in the 2010 US Congressional Election, and they therefore asked us to limit the size of the groups that did not receive the standard “get out the vote” (GOTV) message. As a result, 98% of users were exposed to the social message, while only 1% received the alternative informational message and another 1% received no message. Fortunately, the large number of users means there were still over 600,000 people in each of the 1% groups. Whe